1. Field of the Invention
This invention pertains to the field of closed circuit television (CCTV) and more particularly to a device for permitting two-way transmission of video information and control signals over a single conductive link such as coaxial cable or twisted wire pair.
2. Description of the Prior Art
Closed circuit television systems ordinarily include a control or console panel and monitor mounted remote from a television camera from which a television picture is received. Remote control of the camera is achieved via camera control devices such as lens focus, iris, zoom, and camera attitude (pan and tilt) mechanisms mounted on the camera. These mechanisms, may include a motor mechanically coupled to the lens to vary, in the case of the focus control, for example, the focus of the lens. It is desireable to accomplish control of such mechanisms from the control console to allow a person watching the monitor, referred to herein as the operator, to control the camera. This permits the operator to select the area he wishes to observe. This ability is particularly important in security type installations, since it gives the operator the capability to view selected areas in which security related problems may be occurring.
In most CCTV installations, separate cables are run between the camera and the control console to carry the video signal from the camera to the control console, and to carry the various camera control signals from the control console to the camera. Typically, a five conductor cable carries the focus, zoom and iris control signals, and an eight conductor cable carries the pan and tilt control signals between the camera and control room for complete control over the camera's field of view. Additionally, another cable such as a coaxial cable or twisted pair, hereinafter referred to as a single link, is needed to carry the television video signal.
Such closed circuit installations can be expensive when the control console is located more than a short distance from the camera, due to the labor necessary to install numerous cables as well as the cost of the cables themselves. The cost for wire alone can be very high when long cable runs, for example, in the order of a mile, are needed.
Systems are known in the art in which a single link, such as a coaxial cable, carries both the control signals from the control console and the video signals from the camera. In these single link systems a receiver which is mounted near the camera, receives the camera control signals generated at the control room console and decodes and couples them to the various camera control devices, such as the pan, tilt, zoom, focus and iris motors.
In such systems the control signals for the zoom, focus, iris, pan and tilt etc. functions are encoded and inserted into a portion of the video signal known as the "back porch". The back porch is the portion of a television signal that lies between the trailing edge of a vertical synchronizing pulse and the trailing edge of the corresponding blanking pulse. The back porch is typically of a duration to include 15 to 20 horizontal lines that are not used to carry picture information. The vertical back porch immediately follows the 190 microsecond vertical synchronizing pulse and has a duration of approximately 900 microseconds.
Many different types of television systems, including broadcast and closed circuit systems, use the back porch area to carry information other than picture information. For example, many commercial broadcasters insert teletext, VIR signals or closed captioned signals for the hearing impaired into the 15 to 20 unused lines of the back porch. In the CCTV systems under discussion the inserted information is control signals for the various camera control mechanisms.
The camera control signals can be pulses encoded in any manner, for example, binary, BCD or a simple arithmetic counting of the pulses. In the simplest systems an arbitrary number of pulses, for example, four, will correspond to, for example, pan right, while five pulses could correspond to pan left. Any number of pulses can be assigned to any function within the constraints of the time and frequency limitations of the system to permit encoding of a variety of functions.
The performance of long distance installations is severely dependent upon the type and length of wire used to transmit the video and control signals. The distance over which a video or other signal can be transmitted on a cable is limited by the cable characteristics. Over long distances, the signal tends to deteriorate, since it becomes attenuated, particularly at higher frequencies. For example, transmission distances are limited to approximately 1500 feet when RG59U cable is used. This is insufficient in many installations.
In order to overcome the distance problem, in many CCTV installations intermediate amplifiers are added to equalize and boost the signal so that it can be transmitted over a longer distance. In the single link systems described above, however, the use of such amplifiers heretofore has not been possible since known amplifiers are unidirectional devices. If an amplifier was used to amplify the signal received from the camera it would block all of the control signals since the control signals would be applied to the output of the amplifier. The control signals would stop at the amplifier and not be transmitted to the decoding receiver. Thus, in the known art, the use of intermediate amplifiers has been limited to multi-wire installations.